Nitrogen oxides, carbon monoxide, and hydrocarbons are toxic and environmentally damaging byproducts found in the exhaust gas from internal combustion engines. Methods of catalytically converting nitrogen oxides, carbon monoxide, and hydrocarbons into less harmful compounds include the simultaneous conversion of these by-products (i.e., “three-way conversion” or “TWC”). Specifically, nitrogen oxides are converted to nitrogen and oxygen, carbon monoxide is converted to carbon dioxide, and hydrocarbons are converted to carbon dioxide and water.
It has generally been found that TWC significantly increases the efficiency of conversion of these pollutants and, thus, aids in meeting emission standards for automobiles and other vehicles. In order to achieve an efficient three-way conversion of the toxic components in the exhaust gas, conventional TWC contain large quantities of precious metals, such as Pd (Palladium), Pt (Platinum) and Rh (Rhodium), dispersed on suitable oxide carriers. Because Rh-containing catalysts demonstrate a very high activity for the conversion of NOx, Rh is typically considered to be an essential component of the TWC system. As NO emission standards tighten and Rh becomes scarce (and, thus, more expensive), there is an increasing need for new TWC catalyst compositions which require lower amounts of precious metal catalysts and maintain efficient TWC of exhaust byproducts.
Thus, there remains a need for TWC catalyst formulations that do not contain Rh and that exhibit efficient TWC of exhaust byproducts. There also remains a need for efficient methods of producing such TWC catalyst formulations.